1. Field of the Invention
The present invention relates to a communication apparatus and method that can be used in a mobile terminal. More particularly, the present invention relates to a communication apparatus and method for a dual-mode mobile terminal having a wireless local area network (WLAN) interface and a cellular network interface.
2. Description of the Related Art
Recently, various types of wireless communication networks have been deployed, some of which can be classified into either a cellular network or a WLAN. The most recently-deployed cellular communication networks are typically based on one of Code Division Multiple Access 2000 1x Evolution Data Optimized (CDMA2000 1xEVDO), General Packet Radio Services (GPRS), or Universal Mobile Telecommunication Service (UMTS), Global System for Mobile communication (GSM). The Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards typically govern operation of the typical WLANs.
With the deployment of various communication networks, various wireless local area network access technologies based on the IEEE 802.11x standards have been introduced. The WLAN may serve as a cable replacement of the conventional cable modem and Digital Subscriber Line (xDSL) networks for hot spots and homes without compromising the data rate.
The IEEE 802.11 standards use a media access control (MAC) protocol referred to as Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA).
FIGS. 1A to 1C are schematic diagrams illustrating CSMA/CA based wireless communication.
Referring to FIG. 1A, there are four mobile terminals 1, 2, 3 and 4. The first terminal 1 is located within a radio coverage area of the second terminal 2, and the fourth mobile terminal 4 is located within a radio coverage area of the third terminal 3. Thus there is overlapping coverage areas for multiple terminals.
In order to avoid collision while the second mobile terminal 2 communicates with the third mobile terminal 3, the first and fourth mobile terminals 1 and 4 are restricted from using the same channel as terminals 2 and 3. Such collision avoidance can be achieved with the CSMA/CA mechanism.
Referring to FIG. 1B, the second mobile terminal 2 scans radio channels in search of an available channel. If an available channel is found, the second mobile terminal 2 sends a Request To Send (RTS) frame. The RTS frame is received by the mobile terminals located in the radio range of the second mobile terminal 2, i.e. the first and third mobile terminals 1 and 3. The RTS frame carries information about the destination terminal (the third mobile terminal 3) and the data transmission time period indicated by the a Network Allocation Vector (NAV). Upon receiving the RTS frame, the first mobile terminal 1, which is not the destination terminal, stops using the channel during the period indicated by the NAV value, such that the second terminal 2 can send the data to the third mobile terminal 3 without incurring a data collision with the first mobile terminal 1.
Referring to FIG. 1C, the third mobile terminal 3 sends a Confirm To Send (CTS) frame in response to receiving the RTS frame from mobile terminal 2. However, due to the fact that the second and fourth mobile terminals 2 and 4 are located within the radio coverage area of the third mobile terminal 3, the second and fourth mobile terminals 2 and 4 both respectively receive the CTS frame. The CTS frame carries information identifying the destination is the second mobile terminal 2 and a NAV value. Accordingly, the fourth mobile terminal 4 stops transmitting data (i.e. is blocked) during the period indicated by the NAV value such that the second and third mobile terminals 2 and 3 can communicate without interferences/collisions by transmissions from other terminals.
As described above, in the CSMA/CA based WLAN, the terminals can communicate with each other without having a data collision by securing a competition free period by exchanging the RTS/CTS frames with the terminals located in the radio ranges of the terminals involved in the communication.
However, the conventional WLAN communication system has a drawback in that other terminals that are part of a current communication cannot communicate with other terminals since all the terminals use the same communication channel. Accordingly, as the number of the terminals increases in the network especially at a hot spot, the network throughput becomes significantly degraded as the total number of competition free periods are limited may cause terminals to delay transmission that results in communication failures.